TPO 02 - Desert Formation
NOTE：
主要意思（大概就是主谓宾）用粗体标出；重要的其它用斜体；
【】中的是大致意思，可能与原文有关也可能无关，但不会离题
目的为训练句子/段落总结能力
by original markdown
The deserts, which already occupy approximately a fourth of the Earth's land surface, have in recent decades been increasing at an alarming pace. The expansion of desertlike conditions into areas where they did not previously exist is called desertification【解释了什么是沙漠化】. It has been estimated that an additional one-fourth of the Earth's land surface is threatened by this process【process 指的前面的The expansion】.
Desertification is accomplished primarily through the loss of stabilizing natural vegetation and the subsequent accelerated erosion of the soil by wind and water. In some cases the loose soil is blown completely away【blown/blow sway 吹走】, leaving a stony surface. In other cases, the finer particles 【fine 有精细的意思；更细小的颗粒】may be removed, while 【然而，表转折】 the sand-sized particles are accumulated to form mobile hills or ridges of sand. 【段落：引出沙漠化的原因】
Even in the areas that retain a soil cover, the reduction of vegetation typically results in the loss of the soil's ability to absorb substantial quantities of water【植被的减少导致了土壤吸收水分的能力的丧失】. The impact of raindrops on the loose soil tends to transfer fine clay particles into the tiniest soil spaces, sealing them and producing a surface that allows very little water penetration. 【雨滴的冲击会使得土壤发生转换，导致几乎没有水渗透进去】 Water absorption is greatly reduced; consequently runoff【[水文] 径流】 is increased, resulting in accelerated erosion rates. The gradual drying of the soil caused by its diminished ability to absorb water results in the further loss of vegetation, so that a cycle of progressive surface deterioration is established.【这持续的干旱导致植被的流失，构成了一个循环】 【段落：植被丧失的影响】
In some regions, the increase in desert areas is occurring largely as the result of 【作为......的结果】a trend toward drier climatic conditions. Continued gradual global warming has produced an increase in aridity for some areas over the past few thousand years. The process may be accelerated in subsequent decades if global warming resulting from air pollution seriously increases. 【段落：气候对沙漠化的影响】
There is little doubt, however, that desertification in most areas results primarily from human activities rather than natural processes. The semiarid 【adj. 半干旱的；雨量非常少的】 lands bordering【v. 接壤 ；border 边界】 the deserts exist in a delicate ecological balance and 【The lands】are limited in their potential to adjust to increased environmental pressures. 【半干旱地区有着脆弱的生态系统，and后面接的就是在解释怎么个脆弱法】 Expanding populations are subjecting the land to increasing pressures 【sth be subjecting sth to : sth使sth受到...，be subjected to 一般来表示被动，这是用来主动了】to provide them with food and fuel.【人口的增长使得干旱的地区遭受着越来越大供给这些人口食物以及燃料的压力】 In wet periods, the land may be able to respond to these stresses. 【在没有干旱的时候可能还能够应付这种压力】During the dry periods that are common phenomena along【沿着】 the desert margins, though, the pressure on the land is often far in excess 【过度地】 of its diminished capacity, and desertification results. 【虽然在干旱期间，在沿着沙漠的边缘地带，那是很常见的现象，但是，那种对与干旱地区的压力实在是远远超出了其已经削弱了的（提供食物以及燃料）能力，从而导致了沙漠化。】 【段落：人类活动对于是沙漠化的影响】
Four specific activities have been identified as major contributors to the desertification processes: overcultivation, overgrazing, firewood gathering, and overirrigation. The cultivation of crops has expanded into ** progressively drier regions as population densities have grown. 【随着人口密度的增加，农作物的栽培被扩张到了越来越干旱的地区】 These regions are especially likely to have periods of severe dryness, so that crop failures are common. 【这些地区尤其可能有一段严重的干旱时期，以至于农作物欠收是件常有的事】 Since the raising of most crops necessitates 【necessitate 需要】the** prior removal of the natural vegetation, crop failures leave extensive tracts of land 【which】devoid of a plant cover and susceptible to wind and water erosion. 【由于农作物需要提前清楚掉原来的植物，而欠收的农作物又会留下大片的没有植物覆盖基于容易被风和水腐蚀的土地】
The raising of livestock is a major economic activity in semiarid lands, where grasses are generally the dominant type of natural vegetation. 【家畜也是一个重要原因，在一些干旱的地区，草是主要的原生植被】The consequences of an excessive number of livestock grazing in an area are the reduction of the vegetation cover and the trampling and pulverization 【pulverize vt. 粉碎；使成粉末】 of the soil.【大范围畜牧的后果就是植被覆盖的减少以及土地被践踏以及被损毁】 This is usually followed by the drying of the soil and accelerated erosion. 【紧随其后的就是土壤的干旱以及加速腐蚀】 【段落：家畜对于沙漠化的影响】
Firewood is the chief fuel used for cooking and heating in many countries. The increased pressures of expanding populations have led to the removal 【removal 是名词】 of woody plants so that many cities and towns are surrounded by large areas completely lacking in trees and shrubs. 【人们柴火，以至于他们周围的柴火都很稀缺】The increasing use of dried animal waste as a substitute fuel has also hurt the soil because this valuable soil conditioner and source of plant nutrients is no longer being returned to the land. 【The .. substitute fuel 是主语；燃烧死去的动物对土壤有害，因为土壤滋生的调节系统和植物的营养源不再能够恢复了】【段落：柴火收集对于沙漠化的影响】
The final major human cause of desertification is soil salinization resulting from overirrigation.【由于过度灌溉导致的土壤盐化】 Excess water from irrigation sinks down into the water table. 【太多的水渗到土壤里】 If no drainage system exists, the water table rises, bringing dissolved salts to the surface. 【没有排水系统的话，随着水的增多，将会带走无法溶解的盐分到地面】The water evaporates and the salts are left behind, creating a white crustal layer that prevents air and water from reaching the underlying soil. 【水分蒸发，留下白色的一层，妨碍空气和水接触下面的土壤】【段落：过度灌溉对于沙漠化的影响】
The extreme seriousness of desertification results from the vast areas of land and the tremendous numbers of **people ** 【that are】affected , as well as from the great difficulty of reversing or even slowing the process. 【最严重的是大地的广袤，以及受到影响的巨大人群，还有让人们扭转甚至仅仅是减缓沙漠化的困难】 Once the soil has been removed by erosion, only the passage of centuries or millennia will enable new soil to form. 【土壤的流逝，需要几个世纪几千年来使得土地再生】In areas where considerable soil still remains, though, a rigorously enforced program of land protection and cover-crop planting may make it possible to reverse 【扭转 reserve 保留】 the present deterioration of the surface. 【在土壤十分肥沃的地方，使用严格的手段可能扭转目前恶化的地球表面_】